Did Neil Armstrong and Buzz Aldrin have instructions of what to do if they couldn't take off from the moon?

Probably not. I heard an interview with Buzz Aldrin in which he said people often ask if they carried poison pills. They did not. When you can reach up at any time and crack a valve that will let the air out slowly enough you pass out before anything starts to boil, you don't need poison pills.

In all likelihood, these men--experienced test pilots--would have continued working the problem right up until the moment they died. That's what test pilots do.

President Nixon did have a speech waiting in the event of an Apollo 11 failure, and Gemini 9 command Tom Stafford had an informal talk in which he was advised that if Gene Cernan should die during his spacewalk, he had to bring the body home. He argued this would almost certainly be impossible, and he and Gene have always said they discussed it and agreed that in that event, he would unplug the umbilical and leave the body to reenter after a few weeks.

In fact, Cernan experienced dangerously high heart rates and sweated gallons into his spacesuit owing to lack of EVA experience and an absence of handholds. He ended up cutting his EVA short and needed Stafford's help to crank the hatch shut over his ballooning suit.